When opened in a particular PDF viewer, your exported PDF might show spiky strokes that otherwise look fine in GoodNotes or another PDF viewer:
Unfortunately, there isn't much we can do in this case because it depends on the particular PDF Viewer and apparently not all PDF engines are the same. For example, we see that this issue occurs in Chrome, but not in iPhone/iPad PDF Viewer, Mac Preview, or Adobe Reader. While we take a deeper look at our underlying PDF engine and see what we can improve, as a workaround please choose "Editable" for "PDF Data Format" from the Export popover before tapping Export. One drawback is that you might get a larger file size and your handwriting in the PDF won't be searchable. Alternatively, you can export your document as images and possibly use an online tool to merge them back to a single PDF.